Back pain or discomfort is a common problem, whether due to injury or misalignment of vertebrae, poor posture, an unsuitable bed, or fatigue resulting from extended periods of seated work or driving. Various postures and exercises have been devised for offering relief, such as lying on the floor with feet and calves elevated on the seat of a chair, crunches (lifting the upper back and shoulders from a face-up or face-down prone position, while leaving the hips and lower body on the floor) and other such stretches. Sometimes apparatus has been used to assist or modify the effect of such a stretch or posture, by providing a form on which the spine is bent or curved. It is notable that few activities cause the back to become curved backwardly.